Cyclopentadiene (CPD) and its dimer dicyclopentadiene (DCPD) are highly desired raw materials used throughout the chemical industry in a wide range of products such as polymeric materials, polyester resins, synthetic rubbers, solvents, fuels, fuel additives, etc. Typically, cyclopentadiene is produced as a minor byproduct in liquid fed steam cracking (e.g., naphtha and heavier feed) processes. As steam cracking processes shift to using lighter feed (e.g., ethane and propane feed), less CPD is produced while demand for CPD continues to rise. Cyclopentane and cyclopentene also have high value as solvents while cyclopentene may be used as a monomer to produce polymers and as a starting material for other high value chemicals.
Consequently, there is a need for on-purpose CPD production, i.e., CPD produced as a primary product from a feedstock as opposed to CPD produced as a minor byproduct. U.S. Pat. No. 5,633,421 generally discloses a process for dehydrogenating C2-C5 paraffins to obtain corresponding olefins. Similarly, U.S. Pat. No. 2,982,798 generally discloses a process for dehydrogenating aliphatic hydrocarbons containing 3 to 6, inclusive, carbon atoms. However, neither U.S. Pat. No. 5,633,421 nor U.S. Pat. No. 2,982,798 discloses production of CPD from acyclic C5 hydrocarbons, which are desirable as feedstock because they are plentiful and low cost. Further, many challenges exist in designing an on-purpose CPD production process. For example, the reaction converting C5 hydrocarbons to CPD is extremely endothermic and is favored by low pressure and high temperature but significant cracking of n-pentane and other C5 hydrocarbons can occur at relatively low temperature (e.g., 450° C.-500° C.). Other challenges include loss of catalyst activity due to coking during the production process and further processing is needed to remove coke from the catalyst, and the inability to use oxygen-containing gas to directly provide heat input to the reactor without damaging the catalyst.
From the perspective of storage and shipping, DCPD is easier to handle than CPD as a feed material for subsequent chemical syntheses. DCPD and CPD are fungible in many applications. In certain applications DCPD is preferably used directly in lieu of CPD. For other applications where CPD is needed, DCPD can be thermally depolymerized (aka cracked) via retro-Diels-Alder reaction to CPD at the point of use.
Conventional processes for making CPD typically produce C5 hydrocarbon stream(s) comprising CPD at a modest concentration, acyclic diolefins at significant concentrations, and mono olefins. Because many of the C5 species have close boiling points, form azeotropes, and are reactive at distillation temperatures, CPD recovery from the product mixture via conventional distillation is not industrially feasible. In conventional recovery schemes, CPD is recovered from other C5 hydrocarbons utilizing dimerization process(es) which causes CPD to undergo Diels-Alder reaction to produce DCPD that can easily be separated from the C5 hydrocarbons by conventional distillation. Unfortunately, CPD can also react with other diolefins present in the stream to produce co-dimers, which contaminate the DCPD. Furthermore, reactions involving higher-order oligomers also occur at moderate to high temperatures. These side reactions produce undesirable co-dimers and higher-order oligomers, which necessitate more downstream processing steps, such as repeated, multi-step cracking and dimerization, to produce DCPD with sufficient purity required for many applications. Such processes are expensive, low in yield, and can be prone to fouling. In addition, air and oxygen ingress into the CPD reactor and downstream processes and equipment, which can occur when the first reactor operates at below-atmospheric pressure, is highly undesirable. However, thermodynamic equilibrium constraints suppress conversion of acyclic C5 hydrocarbons to CPD when CPD and/or hydrogen partial pressures are elevated.
Therefore, there is a need for processes and systems for the production of CPD and/or DCPD that address the above described challenges.